Electrochemical sensors for the detection of the presence of a species in a fluid material have existed for quite some time. Such sensors include the Clark cell described in U.S. Pat. No. 2,913,386 issued Nov. 17, 1959. The apparatus disclosed in that patent utilizes a dual electrode structure immersed in an electrolyte and encased at least in part in a membrane which is permeable to a predetermined species. In operation such a device allows the permeation of the species to be detected through the membrane and reduces said species at the cathode. At the same time the anode is oxidized as a results of the electrical and ionic connections between the anode and cathode. These oxidation and reduction reactions generate a current which is measurable and is proportional to the concentration of the species being detected. The Clark cell is a large bulky apparatus and must include a liquid electrolytic medium in which the electrodes are immersed. The Clark apparatus suffers from several disadvantages including consumption of the species being detected during detection, slow response times and alteration of the electrolyte during detection
Some of the above-mentioned disadvantages of the Clark-type electrode cell are avoided by apparatus of the type described in U.S. Pat. No. 3,260,656 issued on July 12, 1966 to James W. Ross, Jr. The Ross apparatus utilizes a sandwich comprising a cathode and an anode with a spacer therebetween. This sandwich is immersed in an electrolyte and is geometrically oriented so that the electrodes are parallel to a membrane which is permeable to the species being measured. The membrane combines with a housing to enclose the cathode-anode combination in an electrolyte. In the Ross-type cell the species being measured is consumed at one electrode and regenerated at the other electrode such that no net consumption of the species being detected occurs. Therefore the Ross sensor does not consume the species being measured as a result of the electrochemical reaction of that species with the electrodes. Whereas the Ross cell effectively overcomes the problems of alteration of the electrodes and/or electrolyte, depletion of the species from the test fluid, and extension of the depletion layer into the test fluid causing stirring and fouling dependence, certain other shortcomings are still evident. Among them is the fact that readings with the Ross-type cell, obtained by measuring the current flow between the electrodes, tend to stablize within a maximum of one minute in accordance with the Ross patent. It has been found that response times of this order are not suitable for many applications. A further disadvantage is that the diffusion layer thickness in the Ross cell is determined by the interelectrode distance which is subject to variation as the assembly is stressed by forces arising from temperature and/or pressure variations. Yet another disadvantage is the cumbersome nature of the layered structure making reliable fabrication of Ross-type devices difficult.
Yet another apparatus for electrolytically detecting a species in a fluid is described in U.S. Pat. No. 4,076,596 issued to Connery et al. on Feb. 28, 1978. The apparatus of Connery et al. includes an insulating substrate and a plurality of fingerlike electrodes deposited on the surface of the substrate in a closely spaced interleaved geometric pattern. The electrodes are covered with a thin film of electrolyte and a permeable membrane. The electrolyte is selected so that the species being measured is generated at one electrode and consumed at the other with no net consumption of the species being detected. The Connery et al. apparatus may include a solid electrolyte deposited on the electrodes. While the Connery et al. apparatus eliminates some of the problems of the Ross-type cell it has several disadvantages of its own. One primary disadvantage of the Connery et al. apparatus is that the solid electrolyte is deposited on top of the electrodes. The electrodes form an irregular surface having high points where the electrodes are present and valleys at the spaces between the electrodes. This makes it difficult to deposit a solid electrolyte coating which will be smooth, consistent, homogeneous and adhere to the electrodes. In addition, the coating of electrolyte will be distorted by changes in humidity and temperature because of the irregular surface upon which it is coated. Another problem with the Connery et al. apparatus is that its response times may be too slow for some applications. This results because of the electrolytic resistance of the electrolyte which forms a barrier between the electrodes and the test fluid. As a result, the species must diffuse through the electrolyte prior to contacting the electrodes. Since the Connery et al. electrolyte is coated onto an irregular surface the electrolyte must be thicker than if it were coated on a flat surface to accomplish a complete coating. Accordingly the electrolytic resistance will be lower but diffusion will be slower and can significantly slow response times.
It is the primary object of the present invention to provide an apparatus for electrolytically detecting a species in a fluid material, which has smooth electrolyte coatings with good repeatability.
It is a further object of the present invention to provide a solid electrolyte layer having excellent adherence to the electrodes.
It is a still further object of the present invention to provide a thinner electrolyte layer to thereby reduce the diffusion resistance of the apparatus.
It is a still further object of the present invention to provide an apparatus having a structure which minimizes stresses on the electrolyte and thereby decreases distortion of the electrolyte as a result of temperature and/or humidity variations.
It is a still further object of the present invention to provide an apparatus for electrolytically detecting a species in a fluid material which is capable of operating at room temperature or temperatures significantly lower than prior art electrochemical sensors.
It is a still further object of the present invention to provide an apparatus for electrolytically detecting a species in a fluid material with a response time which is fast enough for use in applications requiring a very fast response.
These and other objects of the present invention will be apparent to one of ordinary skill in the art from the summary and detailed descriptions which follow.